Yeah, but there were other sides to the dynamic. One time, I was attacking one civ who had previously attacked my vassal state, and systematically taking over their cities.
But some of the enemy cities turned out to have been captured from the vassal, and still were 90% their ethnicity. So after I captured those cities, I got a message saying how the people of those cities longed for a return to the motherland or something. Since I was on track for a cultural victory anyway, I allowed them to return to the vassal state, scoring major diplomatic points in the process.